2-quinoxalinol compounds, such as 6-chloro-2-hydroxyquinoxaline, are well known intermediates for the production of pharmaceutically and agriculturally effective chemicals. These compounds are generally prepared by the selective reduction of 2-quinoxalinol-4-oxides. Selective reduction is necessary so that excessive reduction, resulting in the formation of compounds such as 3,4-dihydro-2-quinoxalinol, can be avoided. This is particularly a problem with halogenated 2-quinoxalinol compounds wherein excessive hydrogenation will eliminate the halogen substituent.
U.S. Pat. No. 4,620,003 to Ishikura discloses a process for the reduction of 2-quinoxalinol-4-oxide compounds to 2-quinoxinol compounds, which process involves reacting a 2-quinoxalinol-4-oxide with hydrazine in the presence of (i) a Raney catalyst (especially Raney nickel or sulfided Raney nickel) and (ii) a alkali metal hydroxide, an alkaline earth metal hydroxide, or ammonium hydroxide. While this process reduces the 2-quinoxalinol-4-oxides in desirable efficiencies, a major drawback in the commercial use of the Ishikura process is the relatively high cost of hydrazine relative to other potential reducing agents, in particular hydrogen.
While U.S. Pat. No. 4,636,562 to Davis discloses a process for preparing 2-chloro-6-haloquinoxaline compounds from the corresponding 4-halo-2-nitroaniline, which process includes a step involving the reduction of 6-chloro-2-hydroxyquinoline-4-oxide to 6-chloro-2-hydroxyquinoline employing hydrogen as a reducing agent (in the presence of an aqueous alkali metal hydroxide solvent and a transition element metal hydrogenation catalyst, preferably Raney nickel), this Davis process is also not desirably commercialized. This is because Davis indicates that hydrogen pressures of 1-4 atmospheres are effective with pressures of 1-2 atmospheres being preferred. It is well known to those of ordinary skill in the art that processes involving gaseous hydrogen at such low pressures are inherently dangerous because there is a risk of air leaking into the system. If hydrogen and oxygen contact the hydrogenation catalyst together, water is formed in an explosive manner.
Sakata et al, "The Facile One Pot Synthesis of 6-Substituted 2(1H)-quinoxalinones", Heterocycles, Vol. 23, No. 1 (1985), disclose a process for the reduction of 6-chloro-2-quinoxalinol-4-oxide to 6-chloro-2-quinoxalinol employing hydrogen as a reducing agent and palladium as a catalyst. However, Sakata et al indicate that an undesirable amount of overly reduced by-product is formed.
Consequently, it would be desirable to possess a process for the selective reduction of 2-quinoxalinol-4-oxides to 2-quinoxalinols safely and efficiently employing hydrogen as a reducing agent.
Accordingly, it is an object of this invention to provide a method of selectively reducing 2-quinoxalinol-4-oxides to 2-quinoxalinols employing hydrogen as a reducing agent.
It is a further object of this invention to provide a high pressure method of selectively reducing 2-quinoxalinol-4-oxides to 2-quinoxalinols employing hydrogen as a reducing agent such that the danger of air leaking into the system is minimized.
The above objects and additional objects will become more fully apparent from the following description and accompanying examples.